The present invention relates to a method and to an apparatus for reducing the crest factor (the ratio of peak to average value) of a signal, for example, a multi-carrier communication signal.
In recent years multi-carrier communication systems have been widely used in particular for XDSL communication systems (Digital Subscriber Line) like ADSL (Asymmetric Digital Subscriber Line) or VDSL (Very High Speed Digital Subscriber Line). FIG. 1 illustrates a schematic block diagram of such a transmission system. A serial data signal a is fed to a serial/parallel converter 1 that converts the serial digital data signal a into data packets with N/2 sub-packets, N being an even number. One data packet is transmitted in parallel to an encoder 2, which assigns each sub packet to a separate carrier frequency and supplies a first digital signal vector to an inverse Fourier transformer 3, which performs an inverse Fourier transformation on this vector and generates a second digital signal vector comprising N samples of a signal to be sent. This second digital signal vector is transmitted to a parallel/serial converter 23, which supplies the elements or samples of the second digital signal vector to a digital filter 24 followed by a digital-to-analog converter 25 and a line driver 26. The generated analog transmit signal is transmitted via a channel 27, whereby noise b is added, symbolized by an adder 28. On the receiver side, the signal is equalized by an equalizer/an analog-to-digital converter 29. Then the signal is decoded by performing the reverse operations of the encoding elements 1 to 23, namely through a serial/parallel converter 30, a Fourier transformer 31, a decoder 32, a slicer 33 and a parallel/serial converter 34.
Such a communication system is for example disclosed in U.S. Pat. No. 6,529,925 B1, the content of which is incorporated by reference herein.
Since the transmit signal is composed of a plurality of different signals having different carrier frequencies and amplitudes and phases being determined by the data signal and thus having no predetermined relationships, the amplitude of the transmit signal has approximately a Gaussian distribution. FIG. 2 illustrates the probability h of the amplitude A of the transmit signal as determined by a simulation for a discrete multitone modulated transmit signal with a Fourier transform block length of 256.
Because of this Gaussian distribution, the crest factor of the signal is rather large, that is, the transmit signal has a rather high maximum amplitude value compared to the effective or average amplitude value. Since both the digital-to-analog and analog-to-digital converters, as well as the line drivers, have to be adapted to handle the whole possible amplitude range, these elements have to be defined accordingly causing additional costs and chip space. It is therefore desirable to reduce the crest factor, that is to reduce the maximum amplitude.
In principle, two different approaches are known to reduce the crest factor.
A first method for reducing the maximum amplitudes disturbs the transmit signal. These methods comprise clipping methods as described for example in U.S. Pat. No. 6,038,261.
A second method for reducing the maximum amplitude without disturbing the signal.
In general, these methods use one or more of the carrier frequencies to modify the transmit signal in order to reduce the maximum amplitude. The carrier frequencies used for this purpose may or may not only partially be used for the actual data transmission.
One of these methods is described in the already cited U.S. Pat. No. 6,529,925 B1. There, the Nyquist frequency is used as a single carrier frequency for correction purposes, that is, the last frequency in the inverse Fourier transform. In an ADSL signal, this frequency is not used for data transmission so that the correction does not influence the transmission capacity. However, the performance of this method is limited since only a single carrier frequency is used for correction. Furthermore, this method is not applicable to VDSL signals since the Nyquist frequency is outside the usable frequency range both for downstream and for upstream transmission.
In U.S. Pat. No. 6,424,681 B1 a method for reducing the crest factor using a plurality of carrier frequencies is disclosed. These carrier frequencies are preferably evenly distributed over the whole usable frequency range. From these carrier frequencies a normalized correction signal, a so-called kernel, is generated which has a “Dirac”-like shape, that is, a shape that comprises a single peak as far as possible. To correct a transmit signal, this correction signal is phase shifted to the position of the maximum of the transmit signal and then scaled with a suitable scaling factor depending on the maximum amplitude of the transmit signal. Then, this correction signal is subtracted from the transmit signal. This can be repeated several times to iteratively correct several maximum or peak values. For transmission systems with a great number of carrier frequencies and consequently a great number of signal values in each frame, like a VDSL transmission system, this method is difficult to realize since it needs a relatively long computation time. Furthermore, through the use of a kernel, the carrier frequencies used for the correction have to comprise both low and high frequencies which, consequently, are not usable for data transmission. The use of low carrier frequencies, on the other hand, leads to a greater loss of transmission capacity since lower carrier frequencies can be modulated with more bits than high carrier frequencies due to the lower damping.